Genetic variation of tight junction structures in intestinal inflammation

Abstract

The epithelial barrier facing the external environment in the gastrointestinal (GI) tract is comprised of several components, including the tight junction (TJ) structures observed for the first time in 1963. TJ structures, which are multiprotein complexes composed of transmembrane proteins and a diverse spectrum of intracellular components, create a primary barrier to diffusion of ions, solutes, and water and they concomitantly prevent permeation of pro-inflammatory factors, such as pathogens, toxins, and antigens. Recent studies suggested that disturbance of epithelial integrity is associated with intestinal inflammatory conditions, such as inflammatory bowel disease (IBD), graft-versus-host disease (GVHD), celiac disease, but also diabetes. The general aim of this thesis was to identify novel genetic variants related to the development of intestinal inflammation with a specific focus on the TJ structures, yielding implications for epithelial integrity and paracellular permeability.

Using a case-control study approach (Swedish cases and controls) in paper I, potential associations were investigated between IBD and three selected genetic markers in the genetic region of CLDN1, CLDN2, and CLDN4 (one marker per gene). The strongest association was observed between Crohn’s disease (CD) and the single nucleotide polymorphism (SNP) marker in the genetic region of CLDN2. The same SNP markers were further investigated using a family-based approach in non-Swedish families, but none of the identified associations from the Swedish case-control approach were confirmed. MORC4 which is located in the same genetic region as CLDN2 was also included in the investigation. A significant association was observed between a non-synonymous SNP in MORC4 and CD in the Swedish case-control cohort.

Similarities between IBD and GVHD include intestinal barrier defects and genetic contributions. GVHD is considered to be multifactorial, where the human leucocyte antigen (HLA) acts as a cornerstone; however, non-HLA genes have been identified in association with the outcome after stem cell transplantation (SCT). By using a case-control approach the relationship between non-HLA polymorphisms and emergence of GVHD as well as overall mortality after SCT was analyzed in paper II. The markers of MORC4, CD14, TLR4, and NOD2 were found to associate with the outcome (overall mortality) after SCT. The SNP marker of CD14 was the only analyzed marker that associated with acute GVHD.

In paper III, the associations between IBD and several TJ genes that encode proteins reported to interact with each other were analyzed in a Swedish population. The strongest associations were observed between IBD and SNP markers in the membrane-associated guanylate kinase inverted genes MAGI2 and MAGI3. The MAGI3 SNP was also associated with ileal MAGI3 expression level in the non-inflamed non-IBD subgroup. Furthermore, no overlap between the expression levels of PTEN in inflamed colonic mucosa from patients with CD and those in the non-inflamed mucosa was detected, suggesting that PTEN is an inflammatory marker in CD.

In paper IV, the genetic associations between microscopic colitis (MC) and TJ genes were analyzed in a Swedish population. The strongest association was identified between a SNP marker in PTEN and MC and also the sub-phenotype collagenous colitis (CC). Furthermore, significant associations were observed between genetic variations of MAGI1 and MC and also between a SNP marker in F11R and CC. Moreover, decreased expression levels of PTEN and MAGI1 were primarily associated to CC and the MC subtype lymphocytic colitis (LC), respectively, in comparison with controls.

In conclusion, genes encoding proteins involved in the regulation of the intestinal epithelial integrity, including those contributing to TJ structures, may predispose individuals to intestinal inflammation, such as IBD and MC. Furthermore, MORC4 may be a predisposing factor for CD and one-year mortality after SCT for hematological malignancies.